Method for measuring the light transmission of a photographic film giving a digitized output

ABSTRACT

Clock pulses are generated at a rate synchronized to scanning of a substance with a light beam. The light is energized in pulses having uniform duration in terms of clock pulses. Output of a photocell responding to incident light is integrated to produce a reference signal of linearly varying magnitude. Magnitude of output of a photocell receiving light passed through the substance is compared with the reference signal, and a difference signal is produced whenever the difference between compared signals has a predetermined sign. During a predetermined uniform portion of every light pulse, clock pulses are counted so long as the difference signal persists, giving a digitized measure of substance transparency/opacity.

llnited States Patent [1 1 Torin METHOD FOR MEASURING THE LIGHTTRANSMISSION OF A PHOTOGRAPHIC FILM GIVING A DIGITIZED OUTPUT Inventor:

AB, 581 88 Linkoping, Sweden Filed: May 28, 1971 Appl. No.: 147,950

References Cited .UNITED STATES PATENTS 3,006,238 10/1961 Eberlinem,356/203 Jan Magnus Torin, c/o Saab-Scania June 26, 1973 PrimaryExaminer-Ronald L. Wibert Assistant Examiner-V. P. McGraw Attorney-IraMilton Jones [5 7 ABSTRACT Clock pulses are generated at a ratesynchronized to scanning of asubstance with a light beam. The light isenergized in pulses having uniform duration in terms of clock pulses.Output of a photocell responding to incident light is integrated toproduce a reference signal of linearly varying magnitude. Magnitude ofoutput of photocell receiving light passed through the substance iscompared with the reference signal, and a difference signal is producedwhenever the difference between compared signals has a predeterminedsign. During a predetermined uniform portion of every light pulse, clockpulses are counted so long as the difference signal persists, giving adigitized measure of substance transparency/opacity.

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SIIEH'J 0F 3 SIGNAL AFIPLITUDE a) IIIIIIIIIIIIIIIIIIIIIIIIIIIIIII- b) lI I I VLF I I I I I I I I I I +I I I I S I /\l l I I I /I I g I I 01 I II I I I I I l I I I I I I I I I I "I I I I I I I I A I I v I I III) VK\I I I L I I M s IIIIII IIIIII IIIIII INVENTOR Jan Mag/nus Turm m ATToQ YMETHOD FOR MEASURING THE LIGHT TRANSMISSION OF A PHOTOGRAPHIC FILMGIVING A DIGITIZED OUTPUT This invention relates to a method and adevice for measuring the light transmission of a photographic film byscanning it with a beamed light inciding the film at a substantiallyright angle during relative movement between film and light point in apredetermined path, during which movement the intensity of the incidentlight as well as the light passing through the film are detected, theratio between the light intensities being a measure of the lighttransmission.

It is possible to rapidly read-in and store large quantities ofinformation on a comparatively small area of a photographic film. Sincethe film has high read-in velocity and a large degree of consolidationit is an attractive memory medium in many applications. This advantageis of course especially available when the information initially appearsin the form of radiation, visible or non-visible.

Reading-out information stored on the film is far more difficult tocarry out than reading in information, and in certain cases ispreferably accomplished manually, e.g. in the case where the presence ofa certain event, e.g. enemy activity, has to be verified. In other casesan automatic reading out is possible and even necessary, e.g. when verylarge quantities of information have to be read out or when certaincharacteristic data must be derived from a large number of film frames.This is the case in crystallographic investigations, for example wherethe presence of radiation maximumsso called reflexes obtained by X-raydiffraction-is photographically recorded. In such investigations a largenumber of film frames are obtained in investigating only one crystal,which film frames have to be evaluated optically and mathematically forinformation about the structure etc. of the crystal.

Several different devices for automatically reading out informationstored on a film are known. One such device which is preferably used forautomatically measuring the light transmission of crystallographicframes is already known from a publication in Journal of ScientificInstruments, volume 43, A computer controlled film scanner by ProfessorS. Abrahamsson. The read-out in this and similar known devices occurs insuch a way that a signal is generated correspondingly to the blackeningor light transparency of the film, in a surface element which is allowedto gradually move over the film in a predetermined path until the entirefilm is scanned. The size of the surface element is selected in view ofthe desired resolution. As a measure of the light transmission, asmentioned in the introduction, the relationship between the incidentlight and the light transmitted through the film is used. To eliminateas far as possible the influence of errors originating from variationsof the intensity of the light the latter is held as constant as possibleand the abovementioned signal is to be considered as an analogue signal.

It is known that analogue treatment requires more sophisticated circuitsthan digital treatment and the risk of disturbances is greater withanalogue than with digital treatment. Since reading-out devices of thementioned kind often work together with a computer, to be capable oftreating the often very large quantities of information, this makes itfurther desirable that the signal corresponding to the blackening havedigital character.

The invention is characterized in that a pulse signal is generated andthat the intensity of the incident light is varied at a lower frequencywhich is in integral ratio to the frequency of the pulse signal, andthat the pulses of the pulse signal are counted during time intervalswhich are synchronized with the intensity variation and whose durationis dependent upon the prevailing ratio of incident light intensity tointensity of light transmitted through the film, so that the number ofcounted pulses is a value of the light transmission.

By measuring the light transmission in this way a more reliable resultcan be obtained and the measurement can be performed by a simpler andcheaper device than it has previously been possible.

How a reader according to the invention is constructed will more clearlyappear from the accompanying drawings, in which:

FIG. 1 is a diagrammatic perspective view, partly in section, showing adevice for measuring in the light transmission of a photographic filmaccording to the method of the invention.

FIG. 2 is a block-diagram showing as an example the manner n which themeasurement and the continued signal treatment occur in a film scanneroperating according to the method of the invention.

FIG. 3 is another block-diagram showing as an example how the ratioforming means according to FIG. 2 can be built up.

FIG. 4 is a diagram illustrating the time sequence of signals whichappear at certain points in the block diagram of FIGS. 2 and 3.

Although the method and the device according to the invention will beexplained in connection with a film scanner of the kind described in theabovementioned publication, the invention must not be considered to belimited to this particular use. On the contary the method of measuringlight transmission according to the invention is usable in manyapplications, e.g. for calculating the degree of non-light-transparentparticles in a gas or a liquid.

In FIG. 1 the numeral 1 designates a film scanner which for the purposeof illustration can be considered as consisting of a mechanical unit 2,an electronic unit 3 and an optical unit. The mechanical unit serves totransport a film 4 which shall be transported in a predetermined path,and during this motion the scanning itself is carried out by the opticalunit, and the calculation of the light transmission is performed bymeans of the electronic unit which is connected to a computer 5. Themechanical and optical units are enclosed in a casin 6.

Ihe mechanical unit 2 consists mainly of a cylindershaped film holder 7having at one end a toothed wheel 8 which is rotatably threaded on atubular shaft 9 that has threads along all of its length and has atoothed wheel 10 rigidly secured at one of its ends. The shaft 9 is inturn rotatably mounted on a tubular shaft 11 that is rigidly attached toone of two opposite sides of the casing 6. The length of the threadedshaft 9 is substantially half the length of the shaft 1 I. The toothedwheels 8 and 10 mesh with toothed wheels 12 and 13 respectively on ashaft 14 which is rotatably mounted in bearings on the opposite sides ofthe housing and the shaft 14 is operatively connected to an electricalmotor 15 which is arranged to rotate the shaft I4l'with a uniformvelocity. When the shaft 14 is rotated, the shaft 9 as well as the filmholder '7 are caused to rotate, both in the same direction. However,there are different ratios between the wheels 8, 12 and l0, 13, so thatthe film holder 7 and the threaded shaft 9 will rotate at differentspeeds. As a consequence of this the film holder 7, while rotating, isthreadly advanced along the shaft 9 a certain distance for eachrevolution. The length of the distance depends on the pitch of thethread and the difference between the above mentioned gear ratios. Apoint fixed relative to the casing 6 will thus have relative motionalong a spiral line on the cylinder film surface. This gives scanninglines parallel to each other in the film plane, and by choosing thepitch and gear ratio in a suitable way the lines can be positioned asclose as required to obtain a desired accuracy of the scanning. By meansof a pulse transducer 16 which is operatively connected to the movableshaft 14 through gearing and which is arranged to give pulses timed withthe rotation of the film holder 7, an unambigous value of the movementof the film 4 is obtained. The pulse transducer 16 is connected to theelectronic unit 3.

The optical unit comprises a light emitting diode 17 and two photocells18, 19. The light from the light emitting diode strikes a tilted halftransparent mirror 20 from which a portion of the light is reflectedagainst the film 4. The film is struck at substantially right angles bythe light, which gives rise to a spotlike light on the film. A portionof the light inciding the film 4 passes through it and is reflectedagainst the photocell 18 by means of a reflecting prism 21, housedinside the tubular shaft 11 along the center of the shaft. A portion ofthe light from the light emitting diode 17 passes through the mirror 20and falls onto the photocell 19. The photocells 18, 19 are arranged togive electric signals which correspond to the intensity of the incidentlight, the output signal designated S of the photocell 18 thuscorresponding to the intensity of the light passing through the film 4while the output signal, designed S of the photocell 19 corresponds tothe intensity of the light inciding the film 4. As previously mentioned,the ratio /5 is a measure of the transmission of the film 4. As morefully shown in FIG. 2 the photocells 18, 19 as well as the lightemitting diode 17 are connected to the electronic unit 3 which comprisescircuits that are arranged in such a way that the measurement can becarried out by the method according to the invention.

In FIG. 2 the numeral 16 designated the pulse generator which isarranged to deliver a pulse signal V whose lapse is shown in FIG. 4,diagram a. According to the invention the output of the pulse generator16 is connected to two parallel channels 23, 24 the channel 23 of whichcomprises a frequency divider 25, an amplifier 26 and the light emittingdiode connected in series. The mission of the frequency divider 25 is toform from the pulse signal V a pulse train V whose frequency is anintegral fraction of the frequency of the signal V From V is received,after amplification in the amplifier 26, a pulse train V with the samefrequency as V and with sufficient amplitude to excite the lightemitting diode 17 to produce a light whose intensity varies in pace withV How V varies as a function of time is shown in FIG. 4 diagram b.

The channel 24 comprises a counter 27 which is connected to the pulsegenerator 16 through an electronic gate 28. The gate 28 is arranged togate the signal V to the counter 27 during time periods the duration ofwhich is calculated by a ratio calculator 29 to correspond to the ratiobetween the incident and the transmitted light, i.e., to the lighttransmission of the film 4. The lapse of gated signal, called R is shownin FIG. 4, diagram g. How the duration of the time periods can beadapted to correspond to the transmission of the film 4 will bedescribed in connection with FIG. 3, which illustrates the ratiocalculator in more detail.

The photocells 18, 19 whose output signals are designated by S and Srespectively, are connected through two amplifiers 30, 31 to the inputsof the ratio calculator 29. After amplification the signals S and S, arerespectively designated 8, and S The signals S and S and of course alsoS and S, are in phase with each other and with V, I-Iow S varies as afunction of time is shown in FIG. 4, diagram d. The output signal of theratio calculator 29 which depends on the ratio 8 /5,, and which controlsthe gate 28, is called V and varies with time as shown in FIG. 4,diagram f. As seen from the following the ratio calculator needs for itsfunction a signal which is in phase with V As mentioned both S, and S,-are in phase with V and thus they are usable, but for reasons of loadcapacity the ratio calculator, as seen from FIG. 2, is connecteddirectly to the frequency divider 25.

The film scanner according to FIG. 2 operates in the following manner.The light emitting diode 17, which as abovementioned is caused to emitlight whose intensity varies with a frequency that in an integral ratioto the frequency of the pulse signal V illuminates the film 4 via thesemi-transparent mirror 20. The photocell 18 produces the signal S whichelectrically reproduces the intensity of the light that is transmittedthrough the film and which varies with the blackening of the film. Bymeans of the mirror 20 and the photocell 19 the signal S representingthe intensity of the light ahead of the film is produced, which signalis to be regarded as a reference signal. After amplification, the twosignals are fed to the radio calculator 29, which is arranged to producethe signal V which contains control pulses whose duration depends on theratio S /S The control pulses open the gate 29 so that the counter 27will be fed with the pulses in the pulse signal to a number thatcorresponds to S /S,,, i.e., to the transmission of the film.

The ratio calculator 29 in FIG. 3 comprises an integrator 32 the inputof which is connected to the output of the amplifier 31 and the outputof which is connected to one of the inputs of a comparator 33 having twoinputs. The second input of the comparator 33 is connected to theamplifier 30. The output of the comparator is connected to one of theinputs of a so called EXCLUSIVE OR-gate 34 having two inputs, the otherof which is connected to a phase shift circuit 35 having phase shift andbeing fed by the signal V from the frequency divider 25.

The output signal of the integrator 32 called 8,, is shown in FIG. 4,diagram d. Since the input signal S, to the integrator is a square wavesignal S,,, will be a triangular signal. The comparator 34 is arrangedto compare the signals S and S with each other and to give a binaryoutput signal V, which is logical zero when S, is greater than 8,, andwhich is logical one when S is less than S,,,. The signal V, is therebygiven the same frequency as S, or S i.e., the same frequency as thelight pulses. It can also be shown that the signal V,, by theabovementioned disparity conditions, obtains a phase shift that, inrelation to a signal which has a phase shift of 90 in relation to V isproportional to S /S Such a signal having a phase shift of 90 relativeto V is attained by the phase shift circuit 35 whose output signal canbe regarded as a binary signal called V and is shown in FIG. 4, diagram0. a

The gate 34 is arranged to compare the signals V and V, and to give abinary output signal which is logical zero when the input signals areequal, i.e., when both of them are logical zero or both of them arelogical one, and which is logical one when the signals are unequal. Theoutput signal of the gate 34, which is the earlier mentioned signal Vbecomes by this a pulse signal with a pulse duration that depends on theratio S /S i.e., the transmission of the film.

By letting the signal V control the gate 28, the counter 27 holds, afterone light pulse, a number that is proportional to the transmission ofthe film in the point that is just being scanned. By letting the counterreceive counting pulses during avariable number of consecutive lightpulses, the contents of the counter will still correspond to thetransmission of the film, but since the measurement is carried outduring a plurality of light pulses the contents are to be regarded as alowpass filtered value of the transmission. More specifically, thecounter 27 can be one that is adapted to count counting pulses V duringa selected number of consecutive light energizing pulses V and to dividethe number of counted pulses by the number of light energizing pulses toprovide an average value of the relative transparency of the film.

What is claimed as my invention is:

1. A method of obtaining a digitized measurement of the relative opacityand transparency of different parts of a substance as the same is beingscanned with a beam of light, which method is characterized by:

A. producing clock pulses at a frequency which is synchronized to therate at which the substance is being scanned;

B. producing a train of successive energizing pulses that aresynchronized to the clock pulses, every energizing pulse having aduration equal to the time required for producing a predeterminedintegral number of clock pulses and the pauses between successiveenergizing pulses likewise having uniform durations in terms of anintegral number of clock pulses;

C. in unison with production of the energizing pulses,

energizing a light source used for scanning;

D. producing a pulsing data signal, the pulses of which have a magnitudethat corresponds to the intensity of light from said source that haspassed through the substance being scanned;

E. during the duration of each energizing pulse producing a referencemagnitude signal having a magnitude that changes linearly at apredetermined rate;

F. comparing the magnitude of each pulse of the data signal with itscoexisting reference magnitude signal, and producing a difference signalwhenever the difference between the compared magnitudes is of apredetermined sign; and G. during a predetermined portion of eachenergizing pulse, which portion is the same for every energizing pulse,counting clock pulses for as long as the difference signal coexists withsaid portion of the energizing pulse. 2. A method of measuring therelative opacity and transparency of different parts of a substance asthe same is being scanned with a beam of light, which method ischaracterized by:

A. producing clock pulses at a frequency which is synchronized to therate at which the substance is being scanned; B. producing a train ofsuccessive energizing pulses that are synchronized to the clock pulses,every energizing pulse having a duration equal to the time required forproducing a predetermined integral number of clock pulses and the pausesbetween successive energizing pulses likewise having uniform durationsin terms of an integral number of clock pulses; C. in unison withproduction of the energizing pulses,

energizing a light source used for scanning; D. producing a pair ofsimultaneously pulsing signals,

1. one of which has a pulse magnitude that corresponds to the intensityof light from said source that has passed through the substance beingscanned, and

2. the other of which has a pulse magnitude that corresponds to theintensity of light at said light source;

E. integrating said other pulsing signal to produce a comparison signalof constantly varying magnitude;

F. continuously comparing the pulse magnitude of each pulse of said onepulsing signal with the then prevailing magnitude of said comparisonsignal, and producing a difference signal whenever the differencebetween the compared magnitudes is of a predetermined sign; and

G. during a predetermined portion of each energizing pulse, whichportion is the same for every energizing pulse, counting clock pulsesfor as long as the difference signal persists, to thus obtain adigitized measure of the relative opacity and transparency of theportion of the substance then being scanned.

3. The method of claim 2, further characterized by:

H. defining the portion of each energizing pulse during which clockpulses are to be counted by producing a train of phase-shifted pulsesignals, each of which is in a predetermined phase relation to anenergizing pulse; and

l. counting clock pulses during each energizing pulse only while aphase-shifted pulse signal and a difference signal simultaneouslycoexist.

t t it t

1. A method of obtaining a digitized measurement of the relative opacityand transparency of different parts of a substance as the same is beingscanned with a beam of light, which method is characterized by: A.producing clock pulses at a frequency which is synchronized to the rateat which the substance is being scanned; B. producing a train ofsuccessive energizing pulses that are synchronized to the clock pulses,every energizing pulse having a duration equal to the time required forproducing a predetermined integral number of clock pulses and the pausesbetween successive energizing pulses likewise having uniform durationsin terms of an integral number of clock pulses; C. in unison withproduction of the energizing pulses, energizing a light source used forscanning; D. producing a pulsing data signal, the pulses of which have amagnitude that corresponds to the intensity of light from said sourcethat has passed through the substance being scanned; E. during theduration of each energizing pulse producing a reference magnitude signalhaving a magnitude that changes linearly at a predetermined rate; F.comparing the magnitude of each pulse of the data signal with itscoexisting reference magnitude signal, and producing a difference signalwhenever the difference between the compared magnitudes is of apredetermined sign; and G. during a predetermined portion of eachenergizing pulse, which portion is the same for every energizing pulse,counting clock pulses for as long as the difference signal coexists withsaid portion of the energizing pulse.
 2. A method of measuring therelative opacity and transparency of different parts of a substance asthe same is being scanned with a beam of light, which method ischaracterized by: A. producing clock pulses at a frequency which issynchronized to the rate at which the substance is being scanned; B.producing a train of successive energizing pulses that are synchronizedto the clock pulses, every energizing pulse having a duration equal tothe time required for producing a predetermined integral number of clockpulses and the pauses between successive energizing pulses likewisehaving uniform durations in terms of an integral number of clock pulses;C. in unison with production of the energizing pulses, energizing alight source used for scanning; D. producing a pair of simultaneouslypulsiNg signals,
 2. the other of which has a pulse magnitude thatcorresponds to the intensity of light at said light source; E.integrating said other pulsing signal to produce a comparison signal ofconstantly varying magnitude; F. continuously comparing the pulsemagnitude of each pulse of said one pulsing signal with the thenprevailing magnitude of said comparison signal, and producing adifference signal whenever the difference between the comparedmagnitudes is of a predetermined sign; and G. during a predeterminedportion of each energizing pulse, which portion is the same for everyenergizing pulse, counting clock pulses for as long as the differencesignal persists, to thus obtain a digitized measure of the relativeopacity and transparency of the portion of the substance then beingscanned.
 3. The method of claim 2, further characterized by: H. definingthe portion of each energizing pulse during which clock pulses are to becounted by producing a train of phase-shifted pulse signals, each ofwhich is in a predetermined phase relation to an energizing pulse; andI. counting clock pulses during each energizing pulse only while aphase-shifted pulse signal and a difference signal simultaneouslycoexist.